Method and apparatus for control of the dry line or for control based on the dry line in a Fourdrinier paper machine

ABSTRACT

The system effects an automatic observation of the dry line on wire of the Fourdrinier paper machine and the control actions based on it. In order to produce an image of the dry line range on wire this is illuminated by means of a large, diffusely illuminating surface (20) of even luminosity. The material on moving wire (10) is imaged by an optoelectric camera (40) on a detector whereby the pulp surface preceding the dry line and being specularly reflecting is found homogeneous and bright while the web surface following it and being diffusely reflecting is found homogeneous but matter and darker. The electric image signal delivered by the detector is conducted to a computer (50) in which the dry line is identified as the borderline of said surfaces of different brightness. Its deviations from target values are determined for both the average value and different values in cross direction and the corresponding actuators are adjusted in order to control the paper web on the basis of deviations observed for its different parts or for its whole breadth. Dry line and values of quantities and deviations characterizing it are also presented on a display terminal, whereby said controls can alternatively be effected by a human observer.

BACKGROUND OF THE INVENTION

The present invention relates to a method for monitoring of the dry lineand for control based upon the dry line in a Fourdrinier paper machine.The invention also relates to an apparatus for carrying out said method.

Essential part of the Fourdrinier paper machine is the plane wire onwhich the dilute wood fibre pulp is fed and on which it settles forminga web. The web formation process essentially determines the quality offinal product, since a major part of the water in pulp is removedthrough the wire, and the position of fibres with regard to each otherdoes not change any more in the dryer part following the wire. The mostimportant actuators which affect web formation and through it thequality of the paper or board are located in advance of the wire or inits immediate neighbourhood.

In order to reach a final product of even quality, it is important thatthe properties of the pulp web are measured as early as possible, i.e.already at the wet end of the paper machine. By means of statedactuators one may then reach a fast control and avoid the delay which ischaracteristic to conventional control based on measurements carried outat the dry end. However, practical methods for direct measurement of thepulp web at the wet end have been almost completely missing until recenttimes. The invention to be described lower down presents a new methodfor measurement at the wet end and control based on it.

The method of measurement to be presented is directed to the dry linewhich is related to the disappearance of water from the surface of thepulp web and is found at the location were the water or liquid (fluid)which behaves like water vanishes from said surface. The part of the webwhich precedes this location can be found glossy or specularlyreflecting, due to the light reflected at places by it, while such agloss cannot be observed on the part following the dry line.

In an industrial paper machine, the dry line is irregular in the crossdirection and at the same time variable also in the machine direction.The gloss of the water surface found at an inspection of the wire is notuniform, but consists of spots which being brighter than theirenvironment transmit light reflecting it from various sources of light,like from lamps of the factory hall, to eyes of the observer. A spotcorresponding to even a single source of light is then indefinite andscattered, because it is not the simple mirror image of that lightsource which is observable to the eye, but a nonuniform, glossy areawhich is limited for its size and has an indefinite boundary line,because the water surface of pulp above the moving wire and pulp layeris not very even and because its local inclination is variable. Theglossy area on the web surface sometimes extends sometimes not to thedry line and the water surface of pulp forms narrow, long peaks whoseobservation is made particularly difficult by the unevenness of thegloss.

Despite of the deficient observing of the dry line, the machine operatorrelies regularly on his observations on it in some of his controlactions, i.e. in local or remote adjustment of manually controllableactuators, also in the case of a paper machine which has been providedwith automatic controls based on quantities measured at dry end or dryerpart. In order to obtain a picture of the dry line in its totality, hehas then to inspect the wire from different directions in order toobserve such, reflecting spots which are limited By the dry line at itsdifferent locations. According to his subjective observations heconcludes the deviation of the dry line from the wanted location, bothfor the average value and in order to adjust the feed flow at differentlocations of the cross direction.

In order to observe the dry line instrumentally, one has in some casesinspected the wire by means of a single, photoelectric detector or anoptoelectric camera which may be conventional television camera or acamera based on semiconductor detector which consists of discreteelements being used in the manner which appears from the British patentNo 1430420 or corresponding U.S. Pat. No. 4,831,641. In theseconnections one has also stated the possibility of using electromagneticradiation which is outside the wavelength range of visible light,analogously with the use of visible light, for observation of physicalobject. The use of these detectors as such does not, however, result ina clear and correct image of dry line, nor in correct values ofquantities which characterize it. This is due to reasons which arealready known from the visual observation of the man and which misleadan instrumental observation. If the number or power of light sources isincreased, difficulties are by no means decreased. On the contrary, thenumbers and contrasts of separate glossy areas and levels of brightnessgrow up and the blinding increases which further hampers instrumentalobservation. In addition to this, the determination of the dry line bycomputer from an indefinite camera signal requires a complicatedcomputer program and results in computations which demand much time, ifit can be carried out at all with an accuracy required by the control ofthe web.

The described difficulties do not not appear in the method according tothe Finnish patent No 75887 or the corresponding U.S. Pat. No.5,011,573. In this method the wire is illuminated in such an angle thatdirect reflections from the surface preceding the dry line are notbrought up and direct reflection of other light sources is alsoprevented. Under such conditions, the part following the dry line whichreflects diffusely the light it receives, is observed, due to this lightwhich it emits, as a brighter part of the pulp web than the partpreceding the dry line.

The last method above detects the dry line in industrial usecontinuously with a good accuracy, as a data set which is renewedrepeatedly. The change of power of illumination in the cross directionof the wire which is present in the method does not essentially hamperthe use of the method, but may require the illuminators to be located alonger way out from the wire in order to reach a more even profile ofillumination. In such a case, the need of illuminative and thereforeelectric power increases, especially if a dark pulp is observed whichafter the dry line absorbs a considerable part of the light. In somecases the structures or auxiliary devices of the paper machine may, fortheir part, prevent a practical installation or maintenance of theequipment implied by the method, if the components to be maintained arelocated e.g. above the wire. The new method to be disclosed in thefollowing produces a more even and at same time not blindingillumination and this way a detection of the dry line and control of theweb at an essentially lower electric power than the method presented bythe patent. It can also be carried out in many such paper machineenvironments to which the patented method does not apply for structuralreasons.

SUMMARY OF THE INVENTION

It is characteristic to the new method that the wire is not at allilluminated directly, but only indirectly, by a large surface whichtransmits light evenly so that bright, blinding spots are not formed onthe wire. The primary light source illuminates or the primary sourcesilluminate this surface which typically is diffusely reflecting. Forformation of an image of the wire, an optoelectric camera is used inwhose image plane a real image is formed of the surface of wire andespecially of the whole area in which the dry line normally appears andto which the indirect illumination is primarily aimed. The camera is solocated that it receives light emitted by the illuminated surface andreflected by the surface preceding the dry line. From the part followingthe dry line it receives a smaller part of that light which, arrivingfrom the illuminated surface, is reflected diffusely from the pulp webfollowing the dry line. If light is transmitted to the detector from thegeneral illuminators of the factory hall or from other light sourceswhich are extraneous to the system, by reflections via the imaged areaof the wire, such disturbing sources are switched off or the reflectionsare prevented with shades.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 presents the wire part of paper machine, the dry line, itsinteresting range of appearance and illumination and camera observationof this range.

FIG. 2 presents the wire, its indirect illumination, rays of lightreflected by the surface preceding the dry line, and the camera.

FIG. 3 presents the wire, the dry line, its normal range of appearanceon the surface of wire, and dimensions of the surface of indirectillumination.

FIG. 4 presents the observation of the dry line in axial direction ofthe paper machine, whereby the primary light source illuminatesobliquely from the side the surface which reflects diffusely light ontothe wire.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

At the manufacture of paper, the pulp arrives from the head box onto themoving wire whose interesting part is, in the method according to theinvention, observed with an optoelectric camera in order to produce animage signal and transfer it to the computer which determines the dryline and controls the actuators of the paper machine. In the typicalembodiment according to FIG. 2 no light arrives from the light source 30directly to the camera 40 nor to the wire 10. Instead of that thediffusely reflecting surface 20 is powerfully and evenly illuminated.This is reached by the use of an appropriate reflector behind and in theneighbourhood of the light source 30. Several light sources may be usedfor the same reason. They with their reflectors may be placed near thedifferent edges of the surface 20, provided that direct radiation oflight from them to the camera or from them to the wire and further on tothe camera is prevented.

The part of wire preceding the dry line reflects light which arrives onit. The smaller is the angle of arrival, the more complete is thereflection. The part following the dry line receives an equalillumination, but since it reflects this diffusely into the totalhalf-space above, it emits in the direction of camera much less lightthan a corresponding surface element at the part preceding the dry line.Therefore the camera finds the part of wire preceding the dry linebrighter than the part following it. The effect is strengthened further,if the web found downstream of the dry line absorbs light. The dry lineis detected from the viewed signal transferred to the computer 50, asthe borderline (set of points) of the brighter early part and darkerlate part, by means of an appropriate edge detection algorithm.

Under illumination in the manner described the camera receives lightfrom the total range of appearance of the dry line within which it iswanted to be observed. If the wire would be illuminated directly bylamps or other separate lights sources, parts of the stated range wouldnecessarily remain dark, as viewed by the camera. The method alsoexcludes the presence of such blinding spots in the scene which arecaused by direct light sources and which would disturb the observationof dry line as they are reflected by the pulp surface to the camera andwould in practice prevent the analysis of the image in computer.

If an area which immediately precedes the dry line on the wire 10,presumably on the same side of the wire as the surface 20 receives lightin a wide angle from various points of the surface 20, the main part ofthis light is refracted into the pulp layer and reflected diffusely byfibres within it or in the fibre layer condensed next to the wire.Considerable part of this light returns to the half-space above the wireand a part of this is accordingly directed to the camera. With regard tothis component of light, the surface preceding the dry line acts almostequally to the surface following it and the absolute difference inbrightness is therefore essentially determined by the light reflecteddirectly from the surface of the lean pulp. It is logical to choose thesize and position of the surface 20 in such a way that the angle a (a₁≧a≧a₂) according to the projection presented in FIG. 2, in which anglethe light which is reflected to the camera from the surface of lean pulpboth leaves the wire and arrives on it, is sufficiently small so that agood difference of brightness is reached within the total range of dryline, and that the height of the surface 20 corresponds to the abovetotal range of the angle; i.e. the surface 20 is to be so high that itin the field of view of the camera 40 and taking the reflection from thewire into account covers at least the same angle of view as the wire.

In the embodiment according to FIG. 2, the primary illuminatorsilluminate the surface 20 from its front side. In order to reach thewanted even, diffuse illumination, the surface 20 has been treated withan suitable agent which may be a white dye or e.g. aluminium bronze orfluorescent substance, but also white paper, cloth or oilcloth aremostly applicable. Diffuse illumination can also be reached through atranslucent, light scattering plate which is illuminated from the backside or to which light is conducted via its edges, or through a platehaving a self-luminescent surface. It is essential that the surface 20emits even, diffuse light at least in the direction of the dry linerange on the wire so that light arrives to all areas of the dry linerange also in the angles a according to FIG. 2. This can be effected, ifa coating material is used which reflects both diffusely anddirectionally and if the inclination of the surface 20 is chosenaccording to the stated aim; except a plane, the surface 20 may be bentor e.g. built of segments.

The optical distance from camera to surface 20 corresponding todifferent values of the departure angle a are, according to aninspection of the vertical projection in FIG. 2, almost equal in thecase of specular reflection from the wire and also in that of diffusereflection. In the latter case each surface element of the mass reflectsdiffusely and emits to the half-space above the wire and therefore alsoto the camera, light which it has received from the whole area ofsurface 20. Therefore the pulp surface preceding the dry line is foundalmost homogeneous for its brightness in the corresponding directionacross the whole wire, and the same is valid with regard to surface ofthe mass following the dry line and to its luminosity.

In the case presented by FIG. 2 the camera sees a trapezoidal area ofthe wire in whose middle range the dry line normally lies. In order toobserve the dry line also at the camera's side of the wire, one has toplace the camera at a suitable distance from the wire and not too closeto its side. The distance can be shortened as needed, if two or moreparallel cameras and an illuminating surface of corresponding dimensionsare used. The influence of the geometry of view on the image transferredto the computer can easily by taken into account at the computationalprocessing of the observed image data, i.e. the observation can becorrected to become a geometrically correct image of the wire and dryline which is needed i.a. at determinations of control actions needed atdifferent part of the cross direction. The accuracy required for thesedeterminations and controls like for selection of the lip screws to becontrolled sets also a practical low limit to the directional angle ofcamera and therefore to the values of angles a₁ . . . a₂. It furtherappears from FIG. 2 that since the camera is located outside the wireand generally the paper machine, its adjustments are easy and itsmaintenance can always be carried out as needed.

The horizontal minimum length and location of the surface 20 aredetermined by the length 11 and location of the interesting area,especially at the side toward the camera. The length of the upper edge12 of surface 20 has to be larger than the measure 11, as seen from FIG.3. For the low edge of the surface a measure 13 is sufficient which isonly slightly larger than the length of the interesting area on wire,especially if the stated low edge is close to the other edge of thewire. According to FIG. 3 the optical distance from camera to surface 20increases only a little at transfer from direction of the centraloptical axis horizontally to sides of surface 20. Therefore theluminosities of the surface preceding the dry line and correspondinglyof surface following the dry line change only minutely with thehorizontal directional angle.

The structure of some paper machines allows for arrangement of theindirect illumination and of observation based on this, in the axialdirection of the machine. The primary sources of light, diffuselyilluminating surface and camera can then be located e.g. in the mannerpresented in FIG. 4. The conditions which determine the positions of theequipment and dimensions of the illuminating surface are analogous withthe conditions which relate to the cross directional system presentedabove. Even another wire of the same multi-wire paper machine and thefibre web on it can then sometimes be used for secondary illumination,if its position and reflectance apply to diffuse illumination of the dryline range of the wire to be observed. In this case one has toadditionally install diffusely reflecting auxiliary surfaces at thesides of the stated other wire, so that the wanted dry line range wouldbe observed for its whole breadth. The corrective calculations which areneeded in order to form a geometrically correct image of the dry lineand its location are easily programmed and executed in such cases aswell. From this point of view it is completely possible, if thestructure of the paper machine and the other equipment in the factoryhall set severe limitations, to position the optical axis of the systemto cross the wire even obliquely.

The optics of the camera 40 form a real image of the dry line range ofthe wire on the electronic detector surface of camera which may be acontinuous surface like that in the conventional television camera tubeor one consisting of discrete elements like that in semiconductorcameras. Because the camera stands relatively far from the wire, itsoptics produce without difficulties an accurate image of the whole dryline, and an even smaller accuracy in depth is sufficient for imaging inthe axial direction of the paper machine. Repeated transfer to computerof the image data expressed in electrical form and the electronichardware needed for it represent previously known technology which canbe implemented by means of commercially available components. Theyinclude the components which differentiate from each other the signalelements which exceed or underpass the luminosity threshold given inelectrical form; even several such thresholds may be present.

In order to detect the dry line, the computer is programmed todistinguish in the image signals which arrive or have beenintermittently stored in the memory, from each other the areas which aredarker or lighter than the given threshold value. Edge detectionalgorithms which are appropriate to this task have been presented intextbooks on image analysis. The image signals can be previouslycorrected by software as needed, e.g. in order to take into accountnonhomogeneity of brightness of the illuminating surface or to eliminatesignals which correspond to field of view extending outside of the wire.

The detected dry line data is compared in the computer with thereference or setpoint data and the control needed by the actuator oractuators is determined on the basis of their difference. Such controlactuators are e.g. the control valves for control of the thick stockflow or of level height in head box or vacuum in suction boxes. Othersuch devices are the components for adjustment of the correspondinglocal control loops which provide for fast feedback control of the meanvalue of dry line by means of a conventional, e.g. proportional orP-control algorithm. The transfer of control signals from computer toactuators represents likewise previously known technology. Also the lipscrews or corresponding components Which are connected to the lip of thehead box slice can be controlled on the basis of differences observed atdifferent points of the cross direction, whereby a difference observedat one point may give an impulse for adjustment of the correspondingscrew and of the nearest other screws, in order to produce such a formof the dry line that the quality characteristics of product become aseven as possible in cross direction. On the basis of the observeddifference one may control in feedforward fashion also actuators locatedat a later place, especially those in the dryer part which affect e.g.heating and through it the moisture. The actuators stated above are justexamples of many such devices whose control can be expediently based onmeasurement of dry line in the described manner, and which already aregenerally used either in manual control or in such automatic controlwhich is based on measurement or observation methods of other kind.Neither the computer is required to have any properties which wouldexceed the abilities of the conventional real-time computer. It canwithout difficulties be programmed to distinguish also such features inthe image which call for special actions or special attention, like apartial or total escape of the dry line from its normal range ofpresence, and to launch alarms, controls and recordings based on this.

The control system described forms expediently a uniform entity with thedescribed observation system, even if it would be located in a separatecomputer which receives measurement data from the computer which detectsthe dry line. Reference or setpoint data are given by the operator ofthe paper machine through the keyboard, but it may come as a digitalsignal also automatically from an outside controller whose operation isbased e.g. on measurements made at the dry end and their observeddeviations from their reference values. A control combined in the lattermanner produces in the steady state an even quality of final product, atthe same time as the dry line control system presented eliminates fastthe effects of dynamic disturbances.

As one control method, reference value signals can be given on basis ofdry line data in feedforward fashion, to the feedback control loops ofthe moisture in paper whose actuators are located in the dryer part ofpaper machine, or signals directly to these actuators, when in bothcases the signals are delayed according to the transport time delay ofthe web. If the feed of pulp to the wire is simultaneously controlled infeedback fashion on the basis of measurements made at the dry end andespecially directed to the dry basis weight, the controls of basisweight and moisture can be accomplished independently of each other orin dependence through the process only, since the fibre composition ofthe web does not change any more after the dry line. The inclusion ofthe feedforward component based on measurement of dry line thereforesimplifies the control and increases its accuracy as compared with thosecontrol methods of basis weight and moisture which presently are ingeneral use.

The dry line detected by the computer, its mean value, sporadicexceptional values and other quantities, features and trends describingthe form of dry line are expendiently reproduced by a display terminalor printer, although this is not necessary with regard to automaticcontrol. However, the paper machine operator has at his disposal anumber of manually adjustable actuators and control devices andadjusting elements of various controllers which he traditionallycontrols, for a large part according to his findings on the dry line.Although the described observation and detection system would not beaccompanied by automatic control, it makes the control of paper machineessentially better in expressing to the controller the dry line and itscharacteristic features as well as deviations from the target values andform, including such features which the operator is in no way able toobserve nor conclude by other means, and in accomplishing thiseconomically, also in such a paper machine environment which istechnologically demanding with regard to installation and maintenance.

I claim:
 1. In a method for monitoring of a dry line and for controlbased on the dry line in a Fourdrinier paper machine having a suctionbox, a head box, and a wire, said head box having a slice through whichpulp is fed onto said wire to form a pulp web, in which the wire and thepulp web on it are illuminated in a region of the dry line and observedoptically by forming a two-dimensional image of a whole normal region ofappearance of the dry line in a camera in which image a first area ofpulp web preceding the dry line and a second area of pulp web followingthe dry line have a different degree of brightness each and which imageis transformed repeatedly to an electrical signal, thresholded andprocessed digitally, wherein on the basis of data on degree ofbrightness transmitted by the electric image signal the location of thedry line is determined as the border line between said areas of pulp weband control actions responsive to the location of the dry line aretaken, said control actions comprising at least one of controllingvacuum in the suction box, controlling level of pulp in the head box,and controlling the slice opening, the improvement comprisingilluminating the wire in said region of appearance of the dry line bymeans of a large surface of even luminosity, said large surface emittinglight diffusely in such a manner that said image is formed optically bymeans of light reflected to the camera by the whole said region ofappearance of the dry line, specularly by said first area and diffuselyby said second area.
 2. A method according to claim 1, wherein saidlarge surface of even luminosity is illuminated with light emitted by atleast one primary source of light.
 3. A method for monitoring of the dryline according to claim 1, characterized in that the control actions areeffected by a human operator on the basis of the location of the dryline.
 4. A method for controlling the dry line according to claim 1,characterized in that control actions include manipulation of slice ofhead box in the paper machine at different points of slice crosssection.
 5. A method for controlling the dry line according to claim 1,wherein said machine further comprises a dryer part, said method furthercomprising measuring the moisture content of the pulp web in the dryerpart and adjusting the location of the dry line on the basis of moisturecontent in the dryer part of the paper machine.
 6. In an apparatus formonitoring of the dry line and for control based on the dry line in aFourdrinier paper machine comprising a suction box, a head box, and awire, said head box having a slice through which pulp is fed onto saidwire to form a pulp web, and means for illuminating said wire and pulpweb on it in a region of appearance of the dry line, an optoelectriccamera for generating image data for the whole normal range ofappearance of the dry line on said wire, a digital computer programmedto determine on the basis of said image data the location of the dryline and means for controlling said pulp web in response to the locationof the dry line, said means for controlling said pulp web comprising atleast one of means for controlling vacuum in the suction box, means forcontrolling level of pulp in the head box, and means for controlling theslice opening, said means for illuminating comprising a large surfaceemitting light diffusely and being illuminated evenly by at least oneprimary source of light, said diffusely emitted light illuminating thewire and pulp web on it in such a manner that said optical image isformed by means of light reflected to the camera by the whole saidregion of appearance of the dry line, specularly by a first area of thematerial surface preceding the dry line and diffusely by a second areafollowing the dry line, whereby more light arrives to a unit area of thedetector from the first area than from the second area.
 7. An apparatusaccording to claim 6, wherein the optoelectric camera is situatedoutside and above the wire and the surface emitting light diffusely issituated such that its mirror image with regard to the wire as seen fromthe camera covers the same sector as the normal range of appearance ofthe dry line seen from the camera.
 8. An apparatus according to claim 7,wherein at least one primary source of light is situated at a side ofthe wire and below its height level, the surface emitting lightdiffusely being at the same side of the wire and above its height level,and the camera at the opposite side of the wire.
 9. An apparatusaccording to claim 7, wherein the primary source of light is situated atan edge of the surface emitting light diffusely so that said surface isilluminated obliquely by said primary source.
 10. An apparatus accordingto claim 7, wherein the surface emitting light diffusely is translucentand the at least one primary source of light is situated behind thesurface emitting light diffusely.
 11. An apparatus according to claim 6,further comprising an alarm, said digital computer comprising means forlaunching said alarm when the location of the dry line at leastpartially escapes said normal range of appearance.